Human speech perception shows remarkable abilities to normalize wide variations in speech sounds such as due to speaker difference and context difference. Among such abilities is perceptual normalization of vocal tract size effects. We have been conducting series of experiments on the developmental study of this ability.

In our previous paper (K. Sakata et al., 1990), we reported the results of experiments comparing the ability of adults and young children aged 3-5. In this paper, preliminary results on the infants aged 5-7 month will be presented. Experiments on infants have been conducted using the technique of conditioned head-turn responses. The experiments consist of two parts, experiment 1 and 2. Experiment 1 was conducted essentially following the procedures developed by Kuhl et al. Based on the experience in the experiment 1, slightly modified procedures were employed in the experiment 2. At the same time, control experiment on the adult subjects were conducted under experimental conditions similar to that of infants.